1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus.
2. Description of the Related Art
Up to now, there are some image forming apparatuses such as a copying machine and a laser beam printer which include a sheet processing apparatus. In the sheet processing apparatus, a discharged sheet is taken therein after an image is formed on the sheet, the sheet is subjected to a binding process in the vicinity of a substantially middle portion of the sheet, and is then subjected to a process such as a half fold to thereby perform saddle stitch binding.
In such the sheet processing apparatus, in a case of performing the saddle stitch binding, a folding process is performed as follows. First, a plurality of sheets conveyed one by one are aligned on a containing tray, the plurality of sheets are subjected to a binding process in the vicinity of middle portions of the sheets, and then the middle portions of the plurality of sheets are collectively inserted into a nip between a pair of folding rollers. Subsequently, a sheet bundle obtained by performing the above-mentioned folding process is discharged toward a lower discharge tray from an outlet provided on a discharge side of the pair of folding rollers with a folded portion being a leading edge.
In some sheet processing apparatuses having a structure described above, a creasing process is further performed with a view to reinforcing the fold line.
For example, in such the sheet processing apparatuses, a plurality of sheets are first aligned in an accumulation part, and then middle portions of the sheets in the conveying direction are stapled, thereby performing the folding process with respect to the sheet bundle by a pair of first folding rollers 83 and 84 shown in FIGS. 20 and 21. Further, the fold line a of a center-folded sheet bundle Sa, which is subjected to the folding process by the pair of first folding rollers 83 and 84, is nipped by a second folding roller 85, and a traveling body 853, which supports the second folding roller 85, is moved in the fold line direction in such the state, thereby reinforcing the fold line a of the center-folded sheet bundle Sa.
It should be noted that, when the second folding roller 85 is being operated, the center-folded sheet bundle Sa is held by the pair of first folding rollers 83 and 84 to render the center-folded sheet bundle Sa immovable. In addition, when traveling on the fold line a, the second folding roller 85 is driven by a motor M4, and a traveling body 853 is driven by a motor M5 (see Japanese Patent Application Laid-Open No. 2003-182928).
However, in the conventional sheet processing apparatus for performing such the folding process, the traveling body 853 and the second folding roller 85 are driven by two different motors M4 and M5, thereby making a structure of the apparatus complicated and increasing a manufacturing cost of the apparatus.
Further, when the fold line a is nipped by the second folding roller 85, the fold line a is nipped in such a manner that the second folding roller 85 moves over the center-folded sheet bundle Sa. Herein, in a case where a movement speed of the traveling body 853 is not set to be equal to a circumferential speed of the second folding rollers 85, a wrinkle or a break occurs in the center-folded sheet bundle Sa, thereby causing a damage such as a trace of a roller on the center-folded sheet bundle Sa. In view of this, it is necessary to make the movement speed of the traveling body 853 equivalent to the circumferential speed of the second folding roller 85. With the structure in which two motors are used, it is necessary to perform a sophisticated control of the traveling body and the roller.
Depending on the number of sheets or the like of the center-folded sheet bundle Sa, there is a possibility that the traveling body 853 is allowed to be reciprocated a plurality of times. In this case, the traveling body 853 changes a direction on the fold line. However, when the traveling body 853 changes the direction on the fold line, in a case where the change of direction by the traveling body 853 and a reverse rotation by the second folding roller 85 are not performed at the same timing, a wrinkle or a break occurs in the center-folded sheet bundle Sa, thereby causing a damage such as a trace of a roller on the center-folded sheet bundle Sa. Also in this case, it is necessary to perform a sophisticated control of the traveling body and the roller to crease the sheet bundle Sa without damaging the center-folded sheet bundle Sa.